1. Field of the Invention
The present invention relates to an improved wide angle passive infrared system for detecting the presence of an infrared source and/or the presence of an infrared source entering, exiting or moving within a specific angular field of view and range.
2. Description of the Related Art
Motion detectors, intrusion alarms, occupancy sensors and other passive infrared radiation detection systems employ an infrared lens-detector system with an electrical output signal which varies by a measurable amount as a source of infrared radiation enters, exits or moves within its angular field of view and range. The detector output electrical signal is amplified and employed, for example, to activate an alarm, switch or other control system. The lens-detector system consists of a one or two-dimensional array of Fresnel lenses on a thin strip or sheet each of which focuses incident infrared radiation in a specific angular range onto a sensitive area of a detector. In the prior art a wide angular field of view is achieved by employing an array of Fresnel lenses on a strip or sheet which protrudes from the front surface of the unit. The protruding sectors collect infrared radiation from peripheral angles.
FIG. 1 is a schematic of the configuration of the lens-detector system for motion detectors, intrusion alarms, occupancy sensors and similar systems according to the prior art. A thin, segmented strip or sheet forming an array 10 covers the entrance aperture and extends to the exterior of the lens-detector system; i.e. exterior to the housing 12. A section of a Fresnel lens 14 is molded or cut into each sector of the strip or sheet. In the schematic twelve sectors are indicated. Each individual Fresnel lens focuses incident infrared radiation at some angle onto one edge of a sensitive area of a detector. For example, the Fresnel lens 14 focuses the beam of infrared radiation indicated onto a sensitive area 16 of a detector 18.
As the angle 20 increases the focal spot moves across the sensitive area 16 of the detector 18 and eventually moves off the opposite edge of the sensitive area 16. The change in the electrical output signal of the detector 18 as a focal spot moves on or off the sensitive area 16 is interpreted as an infrared source moving across one of the critical angles for which the focal spot is on the edge of the sensitive area 16 of the detector 18.
For a single infrared source within the overall field of view of the lens strip or sheet 10 there is a multiplicity of focal spots which move across the sensitive area 16 of the detector 18 as the source moves through the overall field of view of the system. An example of this is illustrated in the schematic of FIG. 2. Incident infrared radiation from the enclosed angular ranges 22, for example, is focused onto the corresponding sensitive area 16 of at least one detector 18 by one sector of the Fresnel lens array 10. Infrared radiation incident from the open angular ranges 24, for example, does not lead to a focal spot on a sensitive area of any detector. Thus the intensity of radiation on a sensitive area of one of the detectors will vary significantly as the infrared source moves into or out of one of the enclosed angular ranges. The resulting detector output signal is processed electronically to activate an alarm, switch or other control system.
The configuration of the Fresnel lens to be exterior to the housing allows radiation detection systems of the prior art to detect radiation over a wide range of angles of incidence 20, including low angles such as angles less than about 30.degree.. As shown in FIG. 1, angle of incidence 20 refers to the remainder (.sup..pi. /2) of the angle from the perpendicular to the surface. The angle of incidence 20 is measured relative to the exposed surface. Heretofore, such exterior positioning of the Fresnel lens may not be aesthetically appealing, and further may be suspectable to damage as well as accidents or injury. For example, a detector positioned for detecting people may be brushed against or otherwise contact such people, including children. As such, the exterior Fresnel lens may cause harm to such people.
In the prior art, the positioning of the Fresnel lens or other mechanisms internal to a housing may be more aesthetically pleasing and less susceptible to damage and injury, but such internal configurations heretofore reduce the range of detection, in which low angles of incidence 20 less than, for example, about 30.degree. are not detectable.